This application relates to the general field of angiography; which is the technique of studying the vascular system by means of X-rays while injecting a "contrast media," typically an iodine based fluid, into the blood vessel or organ to be studied. In order for this process to be maximally successful, the flow from the syringe must be precisely controlled and this control has been the subject of several patent applications. These systems have all exhibited various problems, most of which are eliminated by the system disclosed here. In U.S. Pat. Nos. 3,623,474 and 3,631,847 issued to Heilman et al and Hobbs respectively, a signal directly proportional to flow rate is fed back to produce a flow error signal used to control motor speed. The disadvantages of straight velocity control systems are discussed in Heilman's subsequent U.S. Pat. No. 3,701,345 and include mainly the problems in measuring motor velocity over a wide range. For example, the velocity feedback system originally disclosed included a tachometer which produced an analog feedback voltage. This feedback scheme is later described to drift and be subject to noise at low speeds. To solve this problem, in U.S. Pat. No. 3,701,345 a position feedback system is described. In such a system, a potentiometer is used to sense the position of the plunger. The position signal is compared to a command signal and the difference drives the motor. If the command is a ramp of constant slope, then the motor will move at nearly constant velocity. The position signal from the potentiometer is also used to limit the volume of fluid ejected from the syringe by comparing it to a known position command signal which represents the position at which the injector should stop.
Even this improved control system has its limitations, however. Since position is fed back, the system only guarantees that the average velocity between two positions is such that the plunger is in the right place at the end of a certain time interval. This means that velocity can increase and decrease around this average value as long as the average stays the same. From a control systems point of view, there is an extra integrator in the feedback loop and system stability is compromised. The implications to performance in angiography is that a position control system is not as resistant to factors which might cause velocity to briefly change. Such a factor which is quite significant, is imperfections in the ballscrew mechanism, which is typically used to convert the rotary motion of the motor to the linear motion required to move the plunger. The present injector is of significantly greater power than previous machines by almost a factor of two. The force on the ballscrew is doubled and a larger ballscrew has to be used. This means that the possibility for velocity variations due to ballscrew effects is increased. The inventive control system addresses this problem by controlling velocity directly and incorporates a velocity correction feedback path to maintain velocity control with great accuracy.
Angiographic injectors have typically employed a mechanical means for stopping the plunger in case of failure. Given the increased power of this system, such mechanical means would have to be extremely rugged, making the injector head very large and unwieldy. However, given the power of microprocessors for performing performance tests at extremely high speed, it is possible to design an injector with microprocessor control and eliminate any mechanical means for limiting plunger travel. Given the fact that many users do not use mechanical limits, the fact that they are subject to great stress and may in fact fail themselves and the impossibility to set them for each volume limit in a multi-bolus injection, the use of an "electronic stop" can in fact result in enhanced system reliability.